Drawbacks to Helium??

[lowviz]

We are referring to He %.....

Understood. He diffuses faster and makes slow tissues "faster compartments". Messes with the calculations. There is also the added complication of lowering the O2 concentration. Which of the two creeps out the agencies first? -best if I bow of of this discussion now, sorry.



edit:

My bad first. The OP clearly stated: far more liberal with Helium for dives on the same level (eg. GUE Tech1 certifying for 21/35)

With my best Emily Litella, nevermind...

 

[Kern]

My bad, deleted.

 

[sunkmail]

Do they have any model-based justification for saying 17% He won't contribute to DCS but 18% will?

Just a guess -

Using a blending program, If you have a completely empty tank and fill it to 17% He, and top up with air, you get ~17% O2.

Perhaps the 17% He is an attempt to keep minimum PPO2?

(I could be way off here, I know, but it seems to fit with being overly cautious)

 

[DevonDiver]

18% O2 is the boundary between normoxic and hypoxic mixtures. Hypoxic trimix (<18%) requires more complex dive planning and associated skills... thus a higher level course. I don't think the 18% differentiation is based on DCS risk.

 

[waterpirate]

Helium training used to be just helium period. Now the training is more divided and segmented starting with balloons at age four, and working up to helitrox, rec trimic, adv rec trimix, trimix, hypoxic and normoxic.
If helium is used for elimination of nitrogen for the sake of sobriety above 180fsw the amount of he in any given mix is driven by the divers desired ead for a given dive. If you understand helium at this level of training you realise that any speed limits in regard to he percentages is arbitrary, just numbers on the card. I am sure the agencies can and will defend their choice, but it is still just a number.
Below 180fsw which is not tech 1, it is a whole nother ball game as the o2 percentage drops below 18%, and the gasses ability to sustain life at a shallower depth come into play regardless of the he %.
Eric

 

[lowviz]

Just a guess -

Using a blending program, If you have a completely empty tank and fill it to 17% He, and top up with air, you get ~17% O2.

Perhaps the 17% He is an attempt to keep minimum PPO2?

(I could be way off here, I know, but it seems to fit with being overly cautious)

Yes, exactly what I was thinking.

Start with 100 cuft air, you have about 21cuft O2 and 79cuft inerts.

You can add He to drop both N2 and O2.
Solving for a P02 of 0.17, you need to add 23cuft He: 20.9 / ( 100 + He) = 0.17

So this is 18-19% He. (23/123 x 100%)

Suspiciously close to the "limit", maybe coincidence -don't know...

edit:
But still off-topic, this discussion is only about He concentrations/gradients...

 

[peterbj7]

"Motives" might be a bit strong. Helium is a much misunderstood (and longterm somewhat still experiemental) and some of these misunderstandings shape current course offerings.

Some agencies think beginning technical divers can and will screw up and try to "protect" them with low helium contents. As a general rule these agencies tend to have vague and pretty generous standards (i.e. although individual instructors may be quite rigorous the agency's themselves are not)

Other instructors/agencies provisional far more student's and make them come back for remedial skills work because they expect you to deco dive at a high level regardless of whether you're a beginner or not. These latter agencies tend to have stricter standards and cards issued by them include helium earlier in their training progressions and in more liberal amounts.

The general premise of this post seems to be that those agencies that have higher standards introduce helium earlier, for some unexplained reason. I'm afraid that's not my experience. The agencies I work with recognise that helium is tricky to deal with and require high general diving skills before allowing their students anywhere near the stuff.

And of course there's a major difference between diving with a helium mixture that is normoxic and one that is hypoxic. Not because of the helium but because of the oxygen. But any diving involving helium needs to be accurate and controlled. It is quite possible to get a helium bend underwater, and believe me that isn't something you want.

 

[waterpirate]

Helium bend underwater? I am no doctor nor am I a phd. Either you are bent or your not, You can either follow a schedule or you can not. Care to elaborate and advance my understanding...
Eric

 

[LiteHedded]

Price!!

 

[Blackwood]

18% O2 is the boundary between normoxic and hypoxic mixtures. Hypoxic trimix (<18%) requires more complex dive planning and associated skills... thus a higher level course. I don't think the 18% differentiation is based on DCS risk.

But it a maximum FHe we're talking about, not a minimum FO2. Could be hyperoxic.